I went through the matriculation data for the three highly considered Chicago area prep schools, Chicago Lab, Latin School and Frances Parker, and calculated their HYPSM index, or how many kids attended HYPSM in the past four admission cycles. The results are as follows:
*********LabLatinParker
Harvard672
Yale171110
Princeton7*72
Stanford1136
MIT****920
Total5030*20
UC Lab has a graduating class of 129, Latin has 108, and Parker has 80. With that information, over a four year period, 9.7 percent of UC Lab students go to HYPSM while the ratios for Latin and Parker are 7.0% and 6.3% respectively. Note that in addition to HYPSM, Lab School sends about 13 to 14 kids each year to UChicago. However, these numbers are skewed because a great many professors and other employees at UChicago send their kids to the Lab school (60 percent of the students have some affiliation with the University).
One can argue whether HYPSM placement is a good measure for a high school, but given the expense of these schools and the demographic of the typical full pay parent, it likely is for them.